Designing effective fan coil solutions: Types and Functionality of FCUs

Reducing Cat B FCU fit out waste: Types and Functionality of FCUs 

Designing effective fan coil solutions is key to reducing the waste in construction that can occur between the first category A or ‘Cat A’ fit out and the commercial space customisation known as ‘Cat B’. Effective design also helps to ensure that the fan coils selected offer the most efficient heating and cooling solutions for a specific modern commercial building.  

The issue of waste generation often arises due to potential mismatch between the requirements of Cat A and Cat B fit out. A good example is where providing air conditioning to an original, open-plan Cat A interior space is very different from the more complex airflow picture created after completion of the Cat B fit out. This then often requires reassessment of the M&E design and can lead to the installation of additional fan coil units (FCUs) and the removal of original Cat A FCUs that are no longer in the correct positions or that are oversized. This results in increased waste, increased costs, and increased carbon emissions for the building.

Buildings can have as many as 30 to 40 fit outs during their lifecycle. Therefore, getting the FCU design right at the Cat A fit out stage can help to minimise the impact of future fit outs as the use of the commercial space evolves. Fan coils offer a high level of customisation and complete flexibility of integration with building services. They can deliver a combination of heating, cooling, or ventilation, or indeed all three services. Most commonly, fan coils are specified to improve indoor air quality, regulate temperature, and provide some dehumidification.

In this blog we will look at the types and functionality of FCUs that are available for consideration by specifiers as they look to find the optimum design for commercial spaces. The information is taken from our white paper ‘Reducing waste in development fit outs through effective fan coil solution design’. The white paper includes more detail on fan coil types and their functionality along with advice and guidance on how to optimis Cat A fit out FCU specification to reduce unnecessary waste on your next project.  

How does the fan coil unit distribution system affect its functionality and influence design decisions?

A fan coil unit (FCU) conditions the air within a building by using a water-to-air heat exchanger, a fan to draw air over the heat exchanger and ductwork to distribute the conditioned air. The coil of the heat exchanger can either have low temperature hot water, or chilled water flowing through it. This water is supplied using either a two-pipe or a four-pipe distribution system. 

Two-pipe fan coils

A two-pipe fan coil system contains a single water coil unit connected to two pipes fed by a single source. One pipe supplies water to the coil and the other returns it once it has passed through the coil. The water is from a single source and can either be cold water supplied by a chiller unit or warm water from a boiler or renewable technology. It cannot heat or cool at the same time. The two-pipe system can incorporate a changeover sensor to determine the mode of the system and is known as a two-pipe changeover system.

Four-pipe fan coils

The four-pipe fan coil system has two separate cooling and heating water coils. Each coil has its own set of supply and return pipes meaning that no changeover sensor is required as the system has both warm and chilled water available. It can therefore provide either heating or cooling depending on demand and is suited for spaces where there is likely to be a requirement for both within the building.

Considerations affecting the specification of two-pipe or four-pipe FCUs

There are four considerations that will inform the selection of the most suitable distribution system. The system design should be discussed, including reasons for its selection, sizing, and integration with other components with a specialist. This is especially important for projects that combine energy sources or emitters, as is often the case in mixed use developments and in large scale projects where a variety of HVAC solutions may be implemented.

The four considerations are:

• HVAC requirement - Will the FCU deliver space heating, comfort cooling or both?
• Application - Spatial limitations, the function of the building or design objectives can be an important driver for FCU selection and here flexibility of FCUs is pivotal.
• Building type - This is more pertinent in refurbishments and retrofit but some buildings will be better suited to accommodate new pipework or to adapt to the current pipework.
• Aesthetic and space design - The system design and the choice and integration of FCUs should be holistically integrated with the building and its services. 

Once the criteria have been qualified, the choice of a two-pipe system will be used for heating or cooling only. If both services are required, four pipe systems will likely be specified.

In some instances, two pipe changeovers may be possible where both the heating and cooling provision is delivered using two pipe systems but not simultaneously. The solution can deliver heating in certain periods and then be switched to cooling as required. 

What types of fan coils units are available for specification?

Once the most suitable distribution system has been determined the best type of fan coil that fits in with the spatial and aesthetic demands of the building can be selected.

At the Cat A fit out stage, extensive open plan areas are common and require the specification of larger more powerful FCUs to be able to supply the volume of conditioned air required. Around the perimeter of the building near to windows, FCUs are sized to meet the heat loss and heat gains through the glazing as well as small gains from powered equipment, lighting, and people. In central areas where there is a minimal heat loss, FCUs can be specified as cooling only.

There are four main types of FCU available for specification to match the individual project requirements. They all have similar performance levels and can supply the air conditioning required as either heating and cooling or cooling only. 

There is also a fifth type, consisting of small hybrid FCUs known as cassette units. 

Horizontal Concealed FCUs

Horizontal concealed FCUs are compact and are hidden from view in the void behind suspended ceilings. The ceiling and any insulation provide some sound buffering allowing these units to work slightly harder without compromising acoustic comfort and they are commonly specified in commercial projects.

Horizontal Exposed FCUs

Horizontal exposed FCUs are left in open view, and all parts are visible including controls and ducting. They tend to be specified when a client is seeking a modern industrial look for their building. Due to their exposed nature, both aesthetics and the acoustic performance of the FCUs become more important and should be considered for its effect on the overall design of the space. 

Underfloor FCUs

As many commercial buildings have raised floors the void underneath can be used for services and the installation of underfloor FCUs. They can be specified around the perimeter of the building to help mitigate solar gain from large, glazed areas.

Vertical Concealed FCUs or Vertical Exposed FCUs

Vertical concealed FCUs or vertical exposed FCUs are often used for building refurbishment projects where low ceiling heights can restrict the space available to install FCUs and the option of fitting underfloor units may not be available. They can be positioned high up or low down on a wall.

Cassette FCUs

Cassette FCUs are smaller hybrid fan coil units. To try to overcome the issue of original Cat A FCUs being unsuitable for conditioning smaller spaces, more cassette style units that have a lower output can be specified instead. However, each unit requires pipework and electrical connections, increasing the connection time and resources required as well as the carbon footprint of the fitout. Typically, one FCU can do the same duty as three cassette units. FCUs generally run at 50% of their maximum output whereas cassette units run closer to output limit during normal operation. This makes FCUs better placed to cope with fluctuations in demand and they have spare capacity should additional spaces require conditioning.

Where can I find more information on designing effective fan coil solutions?

More information on designing effective fan coil solutions can be found within our Ability white paper ‘Reducing waste in development fit outs through effective fan coil solution design’ 

We have a vastly experienced team on hand to help you find the best solution for your project. We can offer concise and accurate advice and help you evaluate the real-world implications of your fan coil design options to both reduce waste and meet the air conditioning needs of your customers. 

If you would like to learn more about how we can assist in designing effective fan coil solutions for your next project, please contact us and we will be happy to help.